1. Field of the Invention
The present invention relates to a power supply control circuit. In particular, the present invention relates to a power supply control circuit having a function of protecting an output transistor that controls power supply to a load, against overvoltage which may be superimposed on a power supply line (herein after, referred to as “overvoltage protection function”), and also having a function of protecting the output transistor against reverse connection of a power supply (hereinafter, referred to as “power supply reverse connection protection function”).
2. Description of Related Art
Japanese Unexamined Patent Application Publication No. 2007-028747 (hereinafter, referred to as “Patent Document 1”), for example, discloses a power supply control circuit having an overvoltage protection function. The structure of the power supply control circuit is described with reference to FIG. 1. A power supply control circuit 100 includes a gate charge discharging circuit 108, a gate resistor 107, an output MOS transistor 109, a switch 110, a dynamic clamp circuit 111, and a load 112.
The switch 110 and the clamping diode 111 are connected in series with each other between a gate of the output transistor 109 and a power supply line 101 (connected to a positive terminal of a battery power supply or the like). Further, a ground potential (electrically connected to a negative terminal of the battery power supply) 130 is applied to a gate of the switch 110 as a reference voltage. Thus, the overvoltage protection function is realized. Operations thereof are described in detail in Patent Document 1, so a description thereof is herein omitted.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2007-019812 (hereinafter, referred to as “Patent Document 2”), for example, discloses a power supply control circuit having a power supply reverse connection protection function. The structure of the power supply control circuit is described with reference to FIG. 2. Note that components identical with those shown in FIG. 1 are denoted by the same reference numerals. A transistor 114 serving as a control Nch switch is connected between a power supply line (electrically connected to a negative terminal of a battery power supply) 103, which is an illustrative example of a third power supply, and an output transistor 109. As disclosed in Patent Document 2, an electric charge is applied to the output transistor 109 through a parasitic diode 115 of the transistor 114 when a power supply is reversely connected, with the result that the output transistor 109 is rendered conductive and protected. The transistor 114 functions as a power supply reverse connection protection circuit.
Note that a power supply line 102 connected to one end of the load 112 as shown in FIGS. 1 and 2 and serving as a second power supply is connected to the negative terminal of the battery power supply.
While the circuit of FIG. 1 having the overvoltage protection function and the circuit of FIG. 2 having the power supply reverse connection protection function have been proposed as described above, the present inventor has thought that a power supply control circuit having both the functions has not been realized yet. To realize the power supply control circuit having both the functions, the control switch transistor 114 may be provided, as shown in FIG. 2, between the gate of the transistor 109 shown in FIG. 1 and the power supply line connected to the negative terminal of the battery power supply. This structure has a problem in that, in the case where the load 112 generates a counter electromotive voltage upon non-conduction of the output transistor 109, the switch 110 is not rendered conductive and a predetermined clamp voltage is not applied to the output transistor 109.
The transistor 110 for implementing the overvoltage protection function needs to be rendered conductive based on a counter electromotive force generated by the load 112, when the state of the output transistor 109 changes to a non-conductive state. A mechanism thereof is as follows. A change in voltage to a negative voltage due to the counter electromotive force generated from a load of an output terminal 106 is transmitted to the gate of the output transistor 109 through the transistor 108 and the gate resistor 107, thereby rendering the transistor 110 conductive. When a gate potential of the output transistor 109 is lowered, however, the parasitic diode 115 of the transistor 114, which is provided to implement the power supply reverse connection protection function, is rendered conductive, and the decrease in gate potential of the output transistor 109 is suppressed. As a result, the transistor 110 is inhibited from being rendered conductive.